1. Field of the Invention
The present invention relates to a method and apparatus for measuring convergence with respect to a color cathode ray tube (CRT). More particularly, the invention relates to a method and apparatus for measuring convergence of the Red, Green and Blue (R.G.B.) beams of a color CRT by using a color charge coupled device (CCD) camera wherein the image of the color CRT picked up by the camera has a uniform R.G.B brightness value at all positions of the color CRT screen so as to enable convergence to be more accurately measured and adjusted.
2. Description of the Related Art
Generally, in order to display an image in perfect color on the screen of a color CRT used as a display device, such as a color TV or a color monitor, three electron beams, i.e., a three color (Red, Green, and Blue) electron beam, (hereinafter referred to as R.G.B beam) generated from three corresponding electron guns, must be concentrated at a single pixel point of CRT screen with different incidence angles. This concentration of the three electron beams is referred to as convergence alignment. In general, convergence is defined as a condition in which the electron beams (R.G.B.) of a CRT intersect at a specified point.
There are two kinds of convergence, i.e., static convergence and dynamic convergence. Static convergence refers to control of total misconvergence by adjusting the concentration of the R.G.B beam in the vicinity of a central part of the CRT screen, and dynamic convergence concerns adjustment of the concentration of the R.G.B beam in the vicinity of a peripheral part of the screen. Therefore, in a conventional color CRT, the horizontal deviation magnetic field produced by the deviation yoke mounted between the three electron guns and the screen should be a strong xe2x80x9cpincushion-typexe2x80x9d magnetic field, while the vertical deviation magnetic field should be a strong xe2x80x9cbarrel-typexe2x80x9d magnetic field. Each direction of progression of the three (R.G.B) beams is varied by the horizontal and vertical magnetic fields, so that the three (R.G.B) beams are properly concentrated on the screen.
Misconvergence among the three R.G.B beams generated by the electron gun of CRT can occur because of defects in the deviation yoke and/or CRT, or because of errors associated with a mechanical mechanism or the curvature of the CRT screen. Therefore, when one or more of these causes the three R.G.B beams to deviate from one another and not properly converge, a color deviation occurs.
In the system of the prior art, in order to measure convergence of the R.G.B. beams of color CRT, a large number of optical sensors are mounted on the color CRT. Because convergence is measured with respect to each of the positions at which the optical sensors are mounted, convergence cannot be always measured at a particular position desired by the user. Further, the positions of the optical sensors are changed according to the screen size of the color CRT, so that the time required in making convergence measurements is increased. Moreover, it is difficult to change a completed product model the convergence of which is to be tested.
To solve the problems discussed above with respect to convergence measuring systems using optical sensors, another type of convergence measuring system has been developed which employs a color CCD camera. Such a system is shown in FIG. 1, wherein a color CCD camera 14 is shown which produces images of the patterns on the screen of a CRT 10. Such a convergence measuring device can measure convergence at a desired position irrespective of a screen size, but with such a system there is a difference among R (red), G (green), B (blue) brightness values because of the different distances, indicated at d1-d5 in FIG. 1, between patterns. More specifically, R.G.B brightness values for a pattern in the vicinity of the center of the screen are relatively high, while R.G.B brightness values for a pattern at a peripheral part of the screen are a relatively low.
Such a difference in the R.G.B brightness values result in convergence measurement errors, and to accommodate for this, the prior art adjusts the gain of the color CCD camera or the iris of a lens. The prior method of adjusting the lens iris or camera gain can result in simultaneously brightening the center part and peripheral part of the screen or in simultaneously darkening these parts. It will be understood that in order to provide uniform R.G.B brightness values, the brightness of a relatively bright screen part image should preferably be lowered, and the brightness of a relatively dark screen part image should preferably be raised. However, the prior art cannot provide this differentiation with respect to lowering and raising brightness level, but instead simply increases all of the brightness values of all patterns at the same time, or reduces the brightness of all such patterns at the same time. As a result, if the image of a test pattern picked up by the CCD camera is overly dark, the output signal of the color CCD camera is very sensitive to noise, whereas if the image of test pattern is overly bright, the output signal of the color CCD camera is saturated, making it difficult to discriminate the output signal, and thereby lowering the reliability of the convergence measurement data.
The present invention is directed to an apparatus and method for measuring convergence of the R.G.B. beams of a color CRT which overcomes the problems discussed above.
It is an object of the present invention to provide an apparatus and method for measuring convergence with respect to a color CRT wherein a uniform R.G.B brightness value is provided at all positions of the color CRT screen when a color CCD camera images the color CRT, so that convergence can be more accurately measured and adjusted.
In order to achieve this object, an apparatus is provided for measuring convergence of R.G.B. beams of a color CRT, the apparatus comprising: a pattern generator for generating a test pattern of R.G.B. signals on a screen of a color CRT; a color CCD camera for imaging the test pattern formed on the screen of the color CRT to produce an imaged test pattern; a comparator for comparing R.G.B brightness values of the imaged test pattern of the color CCD camera with a predetermined reference brightness value; and a controller for adjusting, in intensity, each of the R.G.B signals of the test pattern generated by the pattern generator so as to produce a uniform R.G.B brightness value when the R.G.B brightness values of the imaged test pattern are determined by said comparator to be different from said reference brightness value, and for, thereafter, measuring convergence.
Preferably, the controller controls the pattern generator so as to adjust the intensity of the R.G.B signals of each test pattern in order to produce a uniform R.G.B brightness value for all test patterns.
Preferably, each test pattern contacts a crossing point of an overall cross-hatch pattern, is positioned within a respective discrete area defined by crossing lines of the cross-hatch pattern, and is of a square shape.
Advantageously, the reference brightness value is within a level range from xe2x80x9c150xe2x80x9d to xe2x80x9c240xe2x80x9d that divides the R.G.B signals, in a grey level range from xe2x80x9c0xe2x80x9d to xe2x80x9c255.xe2x80x9d
In accordance with a further aspect of the invention, a method is provided for measuring convergence R.G.B. signals of a color CRT, the method comprising the steps of: generating a plurality of test patterns on a screen of the color CRT each having a constant R.G.B brightness value; imaging the test pattern formed on the screen of the color CRT using a color CCD camera to produce an imaged test pattern; and adjusting in intensity each of the R.G.B signals of the test pattern on the basis of the R.G.B brightness values of the imaged test pattern produced by the color CCD camera, and thereafter measuring convergence.
Preferably, the adjusting in intensity of the R.G.B signals comprises comparing R.G.B brightness values of the imaged test pattern with a predetermined reference brightness value, and adjusting in intensity the R.G.B signal of each pattern so as to produce a uniform R.G.B brightness value for all test patterns.
Preferably, as above, each test pattern contacts a crossing point of an overall cross-hatch pattern, is positioned within a discrete area defined by crossing lines of the cross-hatch pattern, and is of a square shape.
Advantageously, as above, the reference brightness value is within a level range from xe2x80x9c150xe2x80x9d to xe2x80x9c240xe2x80x9d that divides the R.G.B signals, in a grey level range from xe2x80x9c0xe2x80x9d to xe2x80x9c255.xe2x80x9d